Handbook of Plant Nutrition - chapter 13 pps

The essential nature of molybdenum for higher plants was first reported by Arnon and Stout in 1939 3.. The observation that plant growth was improved by elements other than these led Arno

13 Molybdenum

Russell L Hamlin

Coggins Farms and Produce, Lake Park, Georgia

CONTENTS

13.1 Historical Information .375

13.1.1 Determination of Essentiality 375

13.1.2 Function in Plants .376

13.1.2.1 Nitrogenase 376

13.1.2.2 Nitrate Reductase .377

13.1.2.3 Xanthine Dehydrogenase .377

13.1.2.4 Aldehyde Oxidase .378

13.1.2.5 Sulfite Oxidase .378

13.2 Diagnosis of Molybdenum Status of Plants 378

13.2.1 Deficiency 378

13.2.2 Excess 379

13.2.3 Molybdenum Concentration and Distribution in Plants .379

13.2.4 Analytical Techniques for the Determination of Molybdenum in Plants .382

13.3 Assessment of Molybdenum Status of Soils .382

13.3.1 Soil Molybdenum Content .382

13.3.2 Forms of Molybdenum in Soils .384

13.3.3 Interactions with Phosphorus and Sulfur .385

13.3.4 Soil Analysis .386

13.3.4.1 Determination of Total Molybdenum in Soil .386

13.3.4.2 Determination of Available Molybdenum in Soil 386

13.4 Molybdenum Fertilizers .387

13.4.1 Methods of Application .387

13.4.1.1 Soil Applications .387

13.4.1.2 Foliar Fertilization 388

13.4.1.3 Seed Treatment .388

13.4.2 Crop Response to Applied Molybdenum 388

References 389

13.1 HISTORICAL INFORMATION

13.1.1 D ETERMINATION OF E SSENTIALITY

Molybdenum was discovered in 1778 by the Swedish chemist, Carl Wilhelm Scheele However, its importance in biological systems was not established until 1930 when Bortels discovered that

molyb-denum was essential for the growth of Azotobacter bacteria in a nutrient medium (1) Subsequently

375

in 1936, Steinberg determined that molybdenum was required for the growth of the fungus

Aspergillus niger (2).

The essential nature of molybdenum for higher plants was first reported by Arnon and Stout in

1939 (3) In earlier experiments, Arnon observed that minute amounts of molybdenum improved the growth of plants in solution culture (4), and that a group of seven heavy metals, including

molybdenum, increased the growth of lettuce (Lactuca sativa L.) and asparagus (Asparagus

o fficinalis L.) (5) Prior to these studies (conducted in 1937 and 1938, respectively) only boron,

cop-per, iron, manganese, and zinc were considered to be micronutrients The observation that plant growth was improved by elements other than these led Arnon to believe that the list of essential ele-ments was incomplete, and prompted him to test whether or not molybdenum was essential for the growth of higher plants (3)

In their studies, Arnon and Stout tested the molybdenum requirement of tomato (Lycopersicon esculentum Mill.) by their newly established criteria for essentiality (6) These criteria were (a) a

deficiency of the essential element prevents plants from completing their life cycles; (b) the requirement is specific to the element, the deficiency of which cannot be prevented by any other element; and (c) the element is involved directly in the nutrition of plants Plants grown in purified solution cultures developed deficiency symptoms in the absence of molybdenum, and symptoms were prevented by adding the equivalent of 0.01 mg Mo L⫺1 to the root medium (6) Normal growth was restored to deficient plants if molybdenum was applied to the foliage, thereby estab-lishing that molybdenum exerted its effect directly on growth and not indirectly by affecting the root environment

13.1.2 F UNCTION IN P LANTS

The transition element molybdenum is essential for most organisms and occurs in more than 60 enzymes catalyzing diverse oxidation–reduction reactions (7,8) Although the element is capable of existing in oxidation states from 0 to VI, only the higher oxidation states of IV, V, and VI are impor-tant in biological systems The functions of molybdenum in plants and other organisms are related

to the valence changes that it undergoes as a metallic component of enzymes (9)

With the exception of bacterial nitrogenase, molybdenum-containing enzymes in almost all organisms share a similar molybdopterin compound at their catalytic sites (7,8) This pterin is a molybdenum cofactor (Moco) that is responsible for the correct anchoring and positioning of the molybdenum center within the enzyme so that molybdenum can interact with other components of the electron-transport chain in which the enzyme participates (7) Molybdenum itself is thought to

be biologically inactive until complexed with the cofactor, Moco

Several molybdoenzymes including nitrogenase, nitrate reductase, xanthine dehydrogenase, aldehyde oxidase, and possibly sulfite oxidase are of significance to plants Because of its involve-ment in the processes of N2fixation, nitrate reduction, and the transport of nitrogen compounds in plants, molybdenum plays a crucial role in nitrogen metabolism of plants (10)

13.1.2.1 Nitrogenase

The observation of Bortels (1) that molybdenum was necessary for the growth of Azotobacter was

the first indication that molybdenum played a role in biological processes It is now well established that molybdenum is required for biological N2fixation, an activity that is facilitated by the

molyb-denum-containing enzyme nitrogenase Several types of asymbiotic bacteria, such as Azotobacter, Rhodospirillum, and Klebsiella, are able to fix atmospheric N2, but of particular importance to

agri-culture is the symbiotic relationship between Rhizobium and leguminous crops (10) Nitrogenases

from different organisms are similar in nature, and they catalyze the reduction of molecular nitro-gen (N2) to ammonia (NH3) in the following reaction (11):

N2⫹8H ⫹⫹8e⫺⫹16ATP→2NH3⫹H2⫹16ADP⫹16Pi

One of the great wonders in nature is how the process of N2fixation takes place biologically at nor-mal temperatures and atmospheric pressure (12), when in the Haber–Bosch process, the same reac-tion performed chemically requires temperatures of 300 to 500°C and pressures of ⬎300 atm (13) According to Mishra et al (11), nearly all nitrogenases contain the same two proteins, both of which are inactivated irreversibly in the presence of oxygen: an Mo–Fe protein (MW 200,000) and

an Fe protein (MW 50,000 to 65,000) The Mo–Fe protein contains two atoms of molybdenum and has oxidation–reduction centers of two distinct types: two iron–molybdenum cofactors called FeMoco and four Fe-S (4Fe-4S) centers The Fe–Mo cofactor (FeMoco) of nitrogenase constitutes the active site of the molybdenum-containing nitrogenase protein in N2-fixing organisms (14) The effect of biological N2fixation on the global nitrogen cycle is substantial, with terrestrial nitrogen inputs in the range of 139 to 170×106tons of nitrogen per year (15) Despite the impor-tance of molybdenum to N2-fixing organisms and the nitrogen cycle, the essential nature of molyb-denum for plants is not based on its role in N2fixation The primary breach of the Arnon and Stout criteria of essentiality (6) is that many plants lack the ability to fix atmospheric N2and therefore do not require molybdenum for the activity of nitrogenase In addition, the process of N2fixation is not essential for the growth of legumes if sufficient levels of nitrogen fertilizers are supplied (11,16)

13.1.2.2 Nitrate Reductase

The essential nature of molybdenum as a plant nutrient is based solely on its role in the NO3⫺ reduc-tion process via nitrate reductase This enzyme occurs in most plant species as well as in fungi and bacteria (12), and is the principal molybdenum protein of vegetative plant tissues (17) However, the requirement of molybdenum for nitrogenase activity in root nodules is greater than the requirement

of molybdenum for the activity of nitrate reductase in the vegetative tissues (18) Because nitrate is the major form of soil nitrogen absorbed by plant roots (19), the role of molybdenum as a functional component of nitrate reductase is of greater importance in plant nutrition than its role in N2fixation Like other molybdenum enzymes in plants, nitrate reductase is a homodimeric protein Each identical subunit can function independently in nitrate reduction (9), and each consists of three functional domains: the N-terminal domain associated with a molybdenum cofactor (Moco), the

central heme domain (cytochrome b557), and the C-terminal FAD domain (7,20) This enzyme occurs in the cytoplasm and catalyzes the reduction of nitrate to nitrite (NO⫺2) in plants (19):

NO3⫺⫹ 2H⫹⫹ 2e2⫺→NO2⫺⫹ 2H2O Nitrate and molybdenum are both required for the induction of nitrate reductase in plants, and the enzyme is either absent (21), or its activity is reduced (22), if either nutrient is deficient In

deficient plants, the induction of nitrate reductase activity by nitrate is a slow process, whereas the induction of enzyme activity by molybdenum is much faster (10) It has been demonstrated that the molybdenum requirement of plants is higher if they are supplied nitrate rather than ammonium (NH⫹4) nutrition (23)—an effect that can be almost completely accounted for by the molybdenum

in nitrate reductase (12)

13.1.2.3 Xanthine Dehydrogenase

In addition to the enzymes nitrogenase and nitrate reductase, molybdenum is also a functional compo-nent of xanthine dehydrogenase, which is involved in ureide synthesis and purine catabolism in plants (8) This enzyme is a homodimeric protein of identical subunits, each of which contains one molecule

of FAD, four Fe-S groups, and a molybdenum complex that cycles between its Mo(VI) and Mo(IV) oxidation states (9,13) Xanthine dehydrogenase catalyzes the catabolism of purines to uric acid (7):

purines→xanthine→uric acid

In some legumes, the transport of symbiotically fixed N2from root to shoot occurs in the form of ureides, allantoin, and allantoic acid, which are synthesized from uric acid (10) Although xanthine

dehydrogenase is apparently not essential for plants (10), it can play a key role in nitrogen metabo-lism for certain legumes for which ureides are the most prevalent nitrogen compounds formed in root nodules (9) The poor growth of molybdenum-deficient legumes can be attributed in part to poor upward transport of nitrogen because of disturbed xanthine catabolism (10)

13.1.2.4 Aldehyde Oxidase

Aldehyde oxidases in animals have been well characterized, but only recently has this molybdoen-zyme been purified from plant tissue and described (24) In plants, aldehyde oxidase is considered

to be located in the cytoplasm where it catalyzes the final step in the biosynthesis of the phytohor-mones indoleacetic acid (IAA) and abscisic acid (ABA) (8) These horphytohor-mones control diverse processes and plant responses such as stomatal aperture, germination, seed development, apical dominance, and the regulation of phototropic and gravitropic behavior (25,26) Molybdenum may therefore play an important role in plant development and adaptation to environmental stresses through its effect on the activity of aldehyde oxidase, although other minor pathways exist for the formation of IAA and ABA in plants (7)

13.1.2.5 Sulfite Oxidase

Molybdenum may play a role in sulfur metabolism in plants In biological systems the oxidation of sulfite (SO3⫺) to sulfate (SO4⫺) is mediated by the molybdoenzyme, sulfite oxidase (10) Although this enzyme has been well studied in animals (27), the existence of sulfite oxidase in plants is not well established Marschner (9) explains that the oxidation of sulfite can be brought about by other enzymes such as peroxidases and cytochrome oxidase, as well as a number of metals and superox-ide radicals It is therefore not clear whether a specific sulfite oxidase is involved in the oxidation

of sulfite in higher plants (28) and, consequently, also whether molybdenum is essential in higher plants for sulfite oxidation

13.2 DIAGNOSIS OF MOLYBDENUM STATUS OF PLANTS

13.2.1 D EFICIENCY

The discovery of molybdenum as a plant nutrient led to the diagnosis of the deficiency in a number

of crop plants, with the first report of molybdenum deficiency in the field being made by Anderson

(29) for subterranean clover (Trifolium subterraneum L.) The critical deficiency concentration in most crop plants is quite low, normally between 0.1 and 1.0 mg Mo kg⫺1 in the dry tissue (12) Symptoms of molybdenum deficiency are common among plants grown on acid mineral soils that have low concentrations of available molybdenum, but plants may occasionally become deficient in peat soils due to the retention of molybdenum on humic acids (19,30) Plants also may be prone to molybdenum deficiency under low temperatures and high nitrogen fertility (31)

Because molybdenum is highly mobile in the xylem and the phloem (32), its deficiency symp-toms often appear on the entire plant This appearance is unlike many of the other essential micronutrients where deficiency symptoms are manifest primarily in younger portions of the plant Molybdenum deficiency is peculiar in that it often manifests itself as nitrogen deficiency, particu-larly in legumes These symptoms are related to the function of molybdenum in nitrogen metabo-lism, such as its role in N2fixation and nitrate reduction However, plants suffering from extreme

deficiency often exhibit symptoms that are unique to molybdenum

Legumes often require more molybdenum than other plants, particularly if they are dependent

on N2 as a source of nitrogen (9) Molybdenum-deficient legumes commonly become chlorotic, have stunted growth, and have a restriction in the weight or quantity of root nodules (33,34) In dicotyledonous species, a drastic reduction in leaf size and irregularities in leaf blade formation (whip-tail) are the most typical visible symptoms, caused by local necrosis in the tissue and insufficient

differentiation of vascular bundles at an early stage of leaf development (35) Marginal and interveinal leaf necrosis is a symptom of extreme molybdenum deficiency, and symptoms are often associated with high nitrate concentrations in the leaf, indicating that nitrate reductase activity is impaired (12) The whiptail disorder is observed often in molybdenum-deficient cauliflower (Brassica oleracea var botrytis L.), one of the most sensitive cruciferous crops to low molybdenum nutrition

(36) In addition, molybdenum-deficient beans (Phaseolus vulgaris L.) often develop scald, where

the leaves are pale with interveinal and marginal chlorosis, followed by burning of the leaf margin (36,37) In molybdenum-deficient tomatoes, lower leaves appear mottled and eventually cup upward and develop marginal necrosis (3) Molybdenum deficiency also decreases tasseling and

inhibits anthesis and pollen formation in corn (Zea mays L.) (38) The inhibition of pollen

forma-tion with molybdenum deficiency may explain the lack of fruit formation in molybdenum-deficient

watermelon (Citrullus vulgaris Schrad.) (9,39).

13.2.2 E XCESS

Most plants are not particularly sensitive to excessive molybdenum in the nutrient medium, and the crit-ical toxicity concentration of molybdenum in plants varies widely For instance, molybdenum is toxic

to barley (Hordeum vulgare L.) if leaf tissue levels exceed 135 mg Mo kg⫺1(40), but crops such as cauliflower and onion (Allium cepa L.) are able to accumulate upwards of 600 mg Mo kg⫺1without exhibiting symptoms of toxicity (41) However, tissue concentrations ⬎500 mg Mo kg⫺1can lead to a toxic response in many plants (42), which is characterized by malformation of the leaves, a golden-yel-low discoloration of the shoot tissues (9), and inhibition of root and shoot growth (43) These symp-toms may, in part, be the result of inhibition of iron metabolism by molybdenum in the plant (12) Toxicity symptoms in plants under field conditions are very rare, whereas toxicity to animals feeding on forages high in this element is well known (44) A narrow span exists between nutritional

deficiency for plants and toxicity to ruminants (45) Molybdenum concentrations ⬎10 mg Mo kg⫺1

(dry mass) in forage crops can cause a nutritional disorder called molybdenosis in grazing rumi-nants (9) This disorder is a molybdenum-induced copper deficiency that occurs when the consumed molybdate (MoO4⫺) reacts in the rumen with sulfur to form thiomolybdate complexes, which inhibit copper metabolism (46)

Agricultural practices that can be used to decrease ruminant susceptibility to molybdenosis include field applications of copper and sulfur The strong depressive effects of SO4⫺on MoO4⫺ uptake can lower the molybdenum concentration in plants to levels that are nontoxic (47) Increasing the copper content of forages through fertilization may also help to reduce molybdenum-induced copper deficiency in animals (46)

13.2.3 M OLYBDENUM C ONCENTRATION AND D ISTRIBUTION IN P LANTS

The requirement of plants for molybdenum is lower than any other mineral nutrient except nickel (Ni) (9) Plants differ in their ability to absorb molybdenum from the root medium (48), and the

sufficiency range for molybdenum in plants varies widely (Table 13.1) Most plants contain

sufficient levels of molybdenum—in the range of 0.2 to 2.0 mg Mo kg⫺1—in their dry tissue, but the difference between the critical deficiency and toxicity levels can vary up to a factor of 104(e.g., 0.1 to 1000 mg Mo kg⫺1dry mass) (9)

The source of nitrogen supplied to plants influences their requirement for molybdenum Nitrate-fed plants generally have a high requirement for molybdenum (66), but there are conflicting reports as

to whether plants supplied with reduced nitrogen have a molybdenum requirement Cauliflower developed symptoms of molybdenum deficiency when grown with ammonium salts, urea, glutamate,

or nitrate, in the absence of molybdenum (20) However, Hewitt (67) suggested that the molybdenum requirement, in the presence of reduced nitrogen, may result from the effects of traces of nitrate derived from bacterial nitrification When cauliflower plants were supplied ammonium sulfate and no

TABLE 13.1

Deficient and Sufficient Concentrations of Molybdenum in Plants

Mo Concentration (mg kg⫺⫺1 dry mass) Crop or Plant Type Plant Part Sampled Deficient Sufficient Reference Agronomic Crops

Alfalfa (Medicago sativa L.) Upper portion of tops; prior to ⬍0.4 0.5–5.0 49, 50

blossom

Cotton (Gossypium hirsutum L.) Fully mature leaves; after bloom 0.6–2.0 55

Peanuts (Arachis hypogaea L.) Upper fully developed leaves ⬍1 0.5–1.0 55, 56

Red clover (Trifolium pratense L.) Total aboveground plants; bloom ⬍0.15 0.3–1.59 50

prior to flowering

end of blossom

Sunflower (Helianthus annuus L.) Mature leaves from new growth 0.25–0.75 52

Tobacco (Nicotiana tabacum L.) Mature leaves from new growth 0.1–0.6 52

Vegetable Crops

Beans (Phaseolus vulgaris L.) Youngest fully expanded leaf; ⬍0.2 0.2–5.0 36

flowering

var capitata)

convar botrytis var botrytis) Aboveground portion of plants; ⬍0.26 0.68–1.49 61

appearance of curd

Cucumber (Cucumis sativus L.) Youngest fully mature leaves ⬍0.2 0.2–2.0 36

onset of blossom

molybdenum under sterile conditions, Hewitt and Gundry (68) found that plants showed no abnor-malities and apparently had no molybdenum requirement On transfer to nonsterile conditions, whip-tail symptoms appeared as a characteristic symptom of molybdenum deficiency Hewitt (17) later stated that molybdenum is of very little importance for some plants if nitrate reduction is not neces-sary for nitrogen assimilation, but that it is impossible to say that an element is not required by plants given the limits of current analytical techniques

Molybdenum is absorbed by plant roots in the form of the molybdate ion (MoO4⫺), and its uptake is considered to be controlled metabolically (19) In long-distance transport in plants, molybdenum is readily mobile in the xylem and phloem (32) The form in which molybdenum is translocated is unknown, but its chemical properties indicate that it is most likely transported as MoO4⫺rather than in a complexed form (9) The proportion of various molybdenum constituents

in plants naturally depends on the quantity of molybdenum absorbed and accumulated in the tissue Molybdenum-containing enzymes, such as nitrogenase and nitrate reductase, constitute a major pool for absorbed molybdenum, but under conditions of luxury consumption, excess molybdenum can also be stored in the vacuoles of peripheral cell layers of the plant (69)

The allocation of molybdenum to the various plant organs varies considerably among plant species, but generally the concentration of molybdenum is highest in seeds (12) and in the nodules of N2-fixing plants (9) However, when molybdenum is limiting, preferential accumulation in root nodules may lead

to considerably lower molybdenum content in the shoots and seeds of nodulated legumes (70) Molybdenum concentrations in leaves have been found to exceed concentrations in the stems of

sev-eral crop species such as tomato, alfalfa (Medicago sativa L.), and soybeans (Glycine max Merr.) (12).

TABLE 13.1 (Continued )

Mo Concentration (mg kg⫺⫺1 dry mass) Crop or Plant Type Plant Part Sampled Deficient Sufficient Reference Fruit Crops

Apple (Malus sylvestris Mill.) Mature leaves from new growth 0.10–2.00 52

Avocado (Persea americana Mill.) Mature leaves from new flush 0.05–1.0 52

ananassa Duch.)

Ornamental Plants

(Impatiens x hybrids)

Poinsettia (Euphorbia Mature leaves from new growth ⬍0.5 0.12–0.5 52, 65

pulcherrima Willd.)

cultivars)

Snapdragon (Antirrhinum majus L.) Mature leaves from new growth 0.12–2.0 52

Trees and Shrubs

Common lilac (Syringa vulgaris L.) Mature leaves from new growth 0.12–4.0 52

Loblolly pine (Pinus taeda L.) Needles from terminal cuttings 0.12–0.56 52

Source: Adapted from U.C Gupta, in Molybdenum in Agriculture, Cambridge University Press, New York, 1997, pp.

150–159 With permission from Cambridge University Press.

13.2.4 A NALYTICAL T ECHNIQUES FOR THE D ETERMINATION OF M OLYBDENUM IN P LANTS

The molybdenum status of crops is often overlooked by the farming community, probably because

of the relatively low crop requirement for molybdenum and because of a lack of education on the necessity of molybdenum in fertility programs In addition, many commercial soil and plant analy-sis laboratories fail to report this nutrient in routine tissue and soil analyses This omission may be partially due to the difficulties in accurately determining the small quantities of molybdenum that are normally present in plant tissues It is possible that many molybdenum deficiencies in crop plants are misdiagnosed as nitrogen deficiency because of the similarity in their deficiency symptoms The two most common methods of molybdenum extraction from plant tissues are dry ashing (71) and wet digestion (72), both of which give similar results (12) Dry ashing is often the preferred method of extraction due to the potential hazards involved with the use of perchloric acid (HClO4) for wet digestion (72) Several analytical techniques have been proposed for the determination of molybdenum in the resulting extracts including the dithiol and thiocyanate colorimetric methods, determination by atomic absorption spectrometry (AAS), graphite furnace atomic absorption spec-trometry (GF-AAS), and by inductively coupled plasma atomic emission specspec-trometry (ICP-AES)

As the detection of molybdenum by ICP-AES is less sensitive than for other elements, this method should be used only for plant tissues suspected of having molybdenum concentrations ⬎1.0 mg Mo

kg⫺1(dry mass) (73,74) The dithiol colorimetric method and the AAS method are probably the most commonly used techniques for determining molybdenum in soil and plant materials (12)

The dithiol method developed by Piper and Beckworth (75) and modified by Gupta and MacKay (76) is more sensitive and precise than other colorimetric methods used for the determi-nation of molybdenum in plant tissues This method is based on precipitation and extraction of a green-colored molybdenum dithiol complex after removal of interfering ions from the test solution (77) The molybdenum concentration is determined by comparing the absorbance of the sample with known standards on a light spectrophotometer The detection limit of the dithiol method is about 20 ng Mo mL⫺1, and the recovery of molybdenum added to the plant material has been greater than 90% (12) Although this method is relatively inexpensive, the procedure may be too tedious and time-consuming for use in many commercial analytical laboratories For procedures of the dithiol method, readers are referred to Gupta (73)

Trace quantities of molybdenum in plant material have been determined by flame (78) or flameless AAS (79) These procedures provide adequate sensitivity for molybdenum and are rela-tively rapid, but are subject to matrix interferences (77) The GF-AAS method (80) improves the accuracy and precision of determining low concentrations of molybdenum, and the procedure is applicable to a range of different plant matrices (73) The detection limits for the determination of molybdenum by AAS using flame and graphite furnace are reported to be 10 and 2 ng mL⫺1, respec-tively (78), and the recovery of molybdenum by these two methods is similar to that of the dithiol colorimetric method, ranging from 92 to 95% (12) For details of the flame and graphite furnace AAS methods, the reader is referred to Khan et al (78) and Gupta (73)

13.3 ASSESSMENT OF MOLYBDENUM STATUS OF SOILS

13.3.1 S OIL M OLYBDENUM C ONTENT

The amount of naturally occurring molybdenum in soils depends on the molybdenum concentrations

in the parent materials Igneous rock makes up some 95% of the Earth crust (81) and contains ∼2 mg

Mo kg⫺1 Similar amounts of molybdenum are present in sedimentary rock (82) The total molybde-num content of soils differs by soil type and sometimes by geographical region (Table 13.2) Soils nor-mally contain between 0.013 and 17.0 mg kg⫺1 total molybdenum (44), but molybdenum concentrations can exceed 300 mg Mo kg⫺1in soils derived from organic-rich shale (83) Large quan-tities of molybdenum also occur in soils receiving applications of municipal sewage sludge (84) or in soils that are polluted by mining activities (46) Most agricultural soils contain a relatively low amount

TABLE 13.2 Molybdenum Content of Surface Soils of Different Countries

a Soils derived from basalts and andesites.

b Data for whole soil pro files.

c Soils from areas of the western states of Mo toxicity to grazing animals.

Source: From A Kabata-Pendias, H Pendias, Trace Elements in Soils and Plants 3rd ed., CRC Press,

Boca Raton, FL 2001, pp 260–267 Copyright CRC Press.

of molybdenum by comparison, with an average of 2.0 mg kg⫺1total molybdenum and 0.2 mg kg⫺1 available molybdenum (19)

Soils derived from granite, organic-rich shale, or limestone, and those high in organic matter are usually rich in molybdenum (85,86), and the available molybdenum content generally increases with alkalinity or fineness of the soil texture (85) In contrast, molybdenum is often deficient in well-drained coarse-textured soils or in soils that are highly weathered or acidic (83,87) The accumulation

of molybdenum varies with depth in the soil, but molybdenum is normally highest in the A horizons

of well-drained soils and is highest in the subsoil of poorly drained mineral soils (83) In soils, molyb-denum can occur in four fractions: (a) dissolved molybmolyb-denum in the soil solution, (b) molybmolyb-denum occluded with oxides, (c) molybdenum as a mineral constituent, and (d) molybdenum associated with organic matter (85)

13.3.2 F ORMS OF M OLYBDENUM IN S OILS

The speciation and availability of molybdenum in the soil solution is a function of pH At water pH

⬎5.0, molybdenum exists primarily as MoO4⫺ (84), but at lower pH levels the HMoO4⫺ and

H2MoO4 forms dominate (44) For each unit increase in soil pH above pH 5.0, the soluble molyb-denum concentration increases 100-fold (88) Plants preferentially absorb MoO4⫺and therefore the molybdenum nutrition of plants can be manipulated by altering soil acidity Soil liming is commonly used to alleviate molybdenum deficiencies in plants by increasing the quantity of plant-available molybdenum in the soil solution (89), but the effect of liming on molybdenum nutrition varies by soil and plant type (Table 13.3) Excessive lime use may decrease the solubility of molybdenum through the formation of CaMoO4(44), but Lindsay (90) suggests that this complex is too soluble to persist

in soils Using lime to change the acidity of a clay loam from pH 5 to 6.5 resulted in greater molyb-denum accumulation in cauliflower, alfalfa (Medicago sativa L.), and bromegrass (Bromus inermis

Leyss.), but molybdenum accumulation was relatively unaffected if plants were grown in a sandy loam (Table 13.3) (87) For plants grown in sandy loam, lime and molybdenum were both required

to significantly increase the molybdenum content of the plant tissue

TABLE 13.3

Effects of Soil pH on Molybdenum Concentration in a Few Crops Grown on Two Soils

Mo concentration (mg kg⫺⫺1 )

Soil pH a No Mo Mo (2.5 mg kg⫺⫺1 ) No Mo Mo (2.5 mg kg⫺⫺1 ) No Mo Mo (2.5 mg kg⫺⫺1 )

Silty clay loam

Culloden sandy loam

a Soil:water ratio 1:2.

Source: From U.C Gupta, in Molybdenum in Agriculture, Cambridge University Press, New York, 1997, pp 71–91.

Reprinted with permission from Cambridge University Press.